1. Field of the Invention
The present invention relates to methods and devices for measuring lengths in a scanning particle microscope, and in particular to a method and device for measuring lengths in such a microscope wherein the specimen is mounted on a specimen stage.
2. Description of the Prior Art
The increasing miniaturization and complexity of integrated circuits requires measurement and observation devices capable of accurately analyzing such small structures. Particularly in the case of integrated circuits, the electrical properties of the circuits are dictated by the geometrical configuration of the circuit integrated structure. In undertaking such measurements and observations, competing technical goals must be balanced. In particular, very high measuring precision is required, however, frequently relatively large measurements must be executed, such measurements being large in comparison to the individual structures being analyzed.
A problem exists in semiconductor technology of measuring relatively large lengths under such conditions. Examples of this measuring problem are the measuring of masks having diameters up to 5 inches (12.70 cm), determination of wafer warp during the manufacturing process, and determining distortion of lithographic structure generating devices. If one assumes that the measuring path amounts, for example, to approximately 10 mm, and that an absolute edge position precision of 0.1 .mu.m is necessary, a relative measuring precision of 10.sup.-6 is required of the measuring method.
Because the field of view of both light microscopes and scanning electron microscopes falls far short of being adequate for the length measurement of such large paths, these microscopes are currently combined with laser beam-controlled microscope stages. In such a system, one edge or end of the path to be measured is brought into coincidence with a mark of the measuring microscope under microscopic observation. Thereafter, the microscope stage on which the specimen is now mounted is displaced until the second portion (the other end) of the path to be measured coincides with the marking of the measuring microscope. The displacement path necessary for this method, which was required in order to shift the entire path to be measured from one end to the other end at the marking of the measuring microscope, is measured with a laser interferometer. This displacement path measured with a laser interferometer, by which the microscope stage is thereby displaced, corresponds to the length of the path to be measured.
Such measuring installations wherein a microscope stage is coupled with a laser interferometer exists both in the combination of a laser stage/light microscope and in the combination of a laser stage-scanning electron microscope for length measurement with a sub-micrometer resolution. Such conventional measuring systems always require a relatively complex laser interferometer for monitoring the microscope stage, making the price of such systems considerably more expensive.